Monoamine oxidase (MAO) will be assayed in the platelets of controls, chronic schizophrenic patients and selected relatives. In contrast to previous studies, different substrates at varying concentrations, to determine the Michaelis Constant (Km) and the maximal velocity (Vmax), will be used. The substrates will include dopamine and tryptamine. Platelet MAO from the immediate relatives of chronic schizophrenics will be studied and those relatives will be followed over a five year period to determine whether relatives with low platelet MAO values are at greater risk to develop the disease than similar relatives with normal platelet MAO values. Standard psychiatric evaluations and conventional MAO enzymatic procedures will be used.